The present invention relates to construction equipment, especially cranes, and the use of tailor welded panels to form beams used in the construction equipment. In one embodiment, tailor welded panels are used to make a boom section for a telescoping boom on a mobile lift crane.
Beams in construction equipment are designed to carry loads. The weight of the beam is often a significant consideration with respect to other design and usage elements of the construction equipment in which the beam is used. For example, the weights of the sections of a telescoping boom are a major factor when designing the rest of the crane. The structural stiffness of a telescoping boom is mainly to resist buckling and bending loads. The stiffness is typically maximized with a boom cross-section having minimum weight in order to increase maximum lift capacity of a crane to which the boom is attached. If the boom section weight can be reduced, the lifting capacity of the crane can usually be increased without having to increase the Gross Vehicle Weight (GVW), strength of the carrier and axle capacity. Thus, there have been many attempts to reduce the weight of the sections of the telescoping boom while maintaining the load that the boom can handle. Many such efforts have involved using high strength steel or other material to make the beam so that the beam has a high strength-to-weight ratio.
In most beams used in construction equipment, the loading on the beam is not uniform throughout all parts of the beams. For example, a beam used in a telescoping boom is often operated at an angle, which produces high bending moments in the beam sections. As a result, the top portions of the beams are in tension, and the bottom portions of the beams are in compression. Because of the way different portions of beams in construction equipment are loaded, efforts to reduce weight have also been directed to forming the beam such that it is thicker in areas where the loads are higher, and thinner material is used in areas where the loads are lower, and putting more material at points that are a greater distance from the axis of the beam to increase the buckling resistance of the beam when it is in compression. For example, in U.S. Pat. Nos. 3,620,579 and 4,016,688, a crane is made with interfitting box-like boom sections that have corners made of thicker steel than the thinner plate material between them to maximize strength and minimize weight. The boom sections in the '579 patent have an elongated corner member at each corner thereof, each corner member having generally normally disposed portions, each portion having an elongated inwardly directed linear step along the outer end thereof forming an elongated linear pocket. The boom sections also have elongated plates having edges extended generally parallel to and adjacent the corner members, with edges located in the pockets in the portions so that they overlap onto the steps. The '688 patent describes a method of making the sections of the telescoping boom by welding angle steel and plate steel members together to form a rectangular boom section. The various sections of the boom fit within each other.
Another consideration that must be taken into account when designing a beam is its cost. The cost is a function of both the material used to make it, and the steps used to form the material into the beam. Using composite materials may result in higher strength-to-weight ratios, but may have higher material costs. Formed beams for telescoping boom sections that have curved sections made by bending the metal multiple times provides higher strength than simple flat sheets, but incurs bending costs, which are high because the boom sections are very long and thus specialized computer controlled equipment with skilled labor are needed to perform the multiple bending operation.
In addition to manufacturing costs, operational costs also have to be taken into account. It might be cost advantageous to spend more money to fabricate a lighter boom in the first place because the crane will have lower operating costs over its life that outweigh a higher initial cost. Balancing manufacturing and operational cost, weight and strength considerations is difficult. Also, in some capacity ranges, initial higher beam costs may be appropriate whereas in other capacity ranges, a lower cost boom construction cost will be suitable and most cost effective over the life of the crane.
Thus there is a need for a beam design that has high strength, low weight and low cost. Also, there is a need for a beam design that allows flexibility to make changes in the design to increase strength for beams to be used in applications where higher strength is needed, while keeping the manufactured beam cost low.